1. Field of the Invention
This invention relates to a method and apparatus for accurately determining on-line, and controlling, the stability of the individual fuel assemblies in a boiling water reactor. More particularly, it relates to selecting the fuel assemblies most likely to exhibit instability and performing a stability analysis based on physical solution of the non-linear conservation equations which takes into account nuclear feedback as well as hydraulic effects on the individual fuel assemblies with the effects of cross-coupling included when appropriate. The invention further relates to determining control action required to return a core with unstable fuel assemblies to stable operation, thus providing an effective on-line expert system in real time.
2. Background Information
Boiling flow instabilities must be considered in the design and analysis of many devices used in energy production. In particular, such instabilities should be avoided in most apparatus of interest. Sufficiently large excursions and/or oscillations from the steady state can affect the efficiency of the process, erode thermal margins, and may cause physical damage to mechanical components.
Flow instabilities are of particular concern in boiling water reactor (BWR) cores. BWR plant operators are under strict Nuclear Regulatory Commission guidelines to be alert for, and to suppress, any flow/nuclear instability. Monitoring is typically done by observing neutron flux signals from local power range monitors (LPRMs), and simultaneously adjusting power/flow conditions to remain below a prespecified core stability limit. In the event that an oscillation is noticed in the LPRM signals, the operator inserts control rods or increases total core flow to attempt to suppress instabilities. Additional control action is then required to bring the plant into a desired configuration.
The LPRMs in a BWR are arranged in strings distributed across the reactor core, and a typical BWR may have 160 such detectors. While this means that the operator must monitor a large numer of readings, there are still many fuel assemblies which are not adjacent to an LRPM. This can lead to a safety problem in the form of fuel damage, and release of fission products could also occur, should a fuel assembly some distance away from an LPRM string undergo sustained regional instabilities. Such instabilities may not be detected by the LPRM signals, and hence go unnoticed. Thus, the practice of controlling instabilities through monitoring LPRM signals is cumbersome at best, and could lead to increased downtime and hence economic loss.
U.S. Pat. No. 4,319,959 suggests a system for supervising stability in a BWR in which flux readings from the LPRMs, and signals regarding such operating conditions as rod position, the flow quantity of recirculation water and the thermal power of the core determined by the heat balance of the plant, are used to determine values for the coolant flow quantity and thermal power in each assembly. Channel stability of each assembly is then determined using an equation which takes into account hydrodynamic factors such as inlet flow velocity, inlet subcooling, heat flux, and mean pressure. However, axial distribution of power, cross flow between channels of subchannels, and nuclear feedback effects are not considered. In one embodiment, a correlation of signals from LPRMs spaced along a channel is used to determine flow rate. Indications of channel stability can be used as a guide for adjusting control rod position to improve stability. Such an approach uses lump parameters for assemblies adjacent LPRMs and can not provide meaningful indications of stability in assemblies remote from an LPRM. It also requires calculation of stability for all of the assemblies to locate any that might be unstable.
It is a primary object of the present invention to provide a method and apparatus for controlling the stability of the fuel assemblies in a boiling water reactor which provides an accurate quantitative evaluation of stability based upon physical laws in real-time for on-line implementation.
It is a another object of the invention to achieve the first object by selecting for the accurate quantitative evaluation of stability a limited number of fuel assemblies most susceptible to instability based upon current observed operating conditions of the core.
It is yet another object of the invention to provide such a method and apparatus which generates and uses, in said selection and evaluation, distributed values of pertinent core parameters.
It is still another object of the invention to provide a method and apparatus which evaluates stability taking into account the nuclear feedback as well as hydrodynamic effects on the stability of the individual fuel assemblies.
Finally it is an object of the invention to provide an expert system environment to the plant operator which suggests ways to obviate unstable operation and also to safely recover from unstable operating modes.